The present invention relates to a simplified process for the production of polyamines containing primary aromatically bound amino groups by the hydrolysis of compounds containing terminal aromatic isocyanate groups.
It is known that aromatic isocyanates can be converted into primary aromatic amides by acid hydrolysis, however, the reaction does not go to completion because the amine formed during hydrolysis further reacts with unreacted isocyanate to form the corresponding urea. This further reaction cannot be suppressed even by using excess, strong mineral acid. One example of such a procedure can be found in Japanese Pat. No. 55 007-827.
German Offenlegungsschrift No. 1,270,046 describes a process for the production of defined primary aromatic amines containing polyalkylene glycol ether segments. In this disclosed process reaction products of aromatic diisocyanates or triisocyanates with polyalkylene glycol ethers and/or polyalkylene glycol thioethers, (preferably those having molecular weights of from 400 to 4000) are reacted with secondary or tertiary carbinols. The product of this reaction is then subjected to thermal cleavage at elevated temperatures in an inert solvent (and optionally in the presence of acid catalysts). Besides the high cleavage temperature, this process is also disadvantageous in that the thermal cleavage of the urethanes is accompanied by the formation of inflammable, readily volatile alkenes which are explosive in admixture with air.
German Auslegeschrift No. 1,694,152 describes the production of prepolymers containing at least two terminal amino groups by reacting hydrazine, aminophenyl ethylamine or other diamines with an NCO-prepolymer of a polyether polyol and polyisocyanate (NCO:NH-ratio=1:1.5 to 1:5). In this disclosed process, unreacted amine must be carefully removed in another step because it catalyzes the reaction with polyisocyanates to a considerable extent (leading to short processing times) and even acts as a reactant.
Another possible method for synthesizing polyamines containing urethane groups is described in French Pat. No. 1,415,317. In this disclosed process, NCO-prepolymers containing urethane groups are converted with formic acid into the N-formyl derivatives which are hydrolyzed to form terminal aromatic amines. The reaction of NCO-prepolymers with sulfamic acid according to German Offenlegungsschrift No. 1,555,907 also yields compounds containing terminal amino groups. Relatively high molecular weight prepolymers containing aliphatic, secondary and primary amino groups are obtained according to German Offenlegungsschrift No. 1,215,373 by reacting relatively high molecular weight hydroxyl compounds with ammonia under pressure at elevated temperature in the presence of catalysts. U.S. Pat. No. 3,044,989 describes production of such prepolymers by reacting relatively high molecular weight polyhydroxyl compounds with acrylonitrile, followed by catalytic hydrogenation. Relatively high molecular weight compounds containing terminal amino groups and urethane groups may also be obtained by reacting NCO-prepolymers with enamines, aldimines or ketimines containing hydroxyl groups, followed by hydrolysis (German Auslegeschrift No. 2,546,536 and U.S. Pat. No. 3,865,791). Another possibility for synthesizing aromatic polyamines containing urethane and ether groups is the opening of the ring which occurs in the reaction of the isatoic acid anhydride and diols. Polyamines of this type are described, for example, in U.S. Pat. No. 4,180,644 and in German Auslegeschriften Nos. 2,019,432; 2,619,840; 2,648,774 and 2,648,825. The poor reactivity of aromatic ester amines obtained in this way is a disadvantage in numerous applications.
The reaction of nitroaryl isocyanates with polyols, followed by reduction of the nitro groups to aromatic amino groups is described in U.S. Pat. No. 2,888,439. The main disadvantage of this process is the high cost of the reduction step.
It is also known that certain heteroaromatic isocyanic acid esters can be converted into heteroaromatic amines by hydrolysis in a pH-basic medium. Unfortunately, the hydrolysis conditions described by H. John in J. Prakt. Chemie 130, 314 et seq and 332 et seq (1931) for two specific, heteroaromatic monoisocyanic acid esters are totally unsuitable for the conversion of polyisocyanates into aliphatic and/or aromatic amines and they are also dangerous.
Multistage processes for the production of polyamines have also been developed. In such processes, NCO-prepolymers are hydrolyzed with excess base (alkali hydroxides) to form carbamates. The carbamate-containing mixture is then acidified with mineral acids or ion exchanger resins in an equivalent quantity or in a quantity exceeding the quantity of base to decompose the carbamates. Excess quantities of acid may be neutralized with a base and the polyamines subsequently isolated.
From the above-described processes it can be readily appreciated that known processes for the production of polyamines are complicated. It would therefore be desirable to develop a commercially workable process which is technically simple and efficient.
Previous attempts to obtain satisfactory yields of polyamines by direct hydrolysis of polyisocyanates in a single stage have been unsuccessful. Instead of the desired hydrolysis products, the products obtained are only partially homogeneous, they do not flow freely and they contain a large number of urea groups but very few, if any, amino groups (see Comparison Examples infra). One possible explanation for the poor quality of these products is that the isocyanate/amine reaction is considerably faster than the isocyanate/water reaction.
German Auslegeschrift No. 1,235,499 teaches that solutions of NCO-prepolymers in dimethyl formamide could be converted with substantially equivalent quantities of water (80 to 120% of the theoretical) into highly viscious solutions which are suitable for spinning elastane spandex filaments or for coatings.
It is also known that isocyanates react with dialkyl formamide to form formamidines (H. Ulrich et al, J. Org. Chem. 33, 3928-3930 (1968)).